The invention relates to a process for isolating oligosaccharides which contain a substituent derived from sialic acid (hereinafter: sialic acid containing oligosaccharides) and preferably sialyllactose containing compositions and especially such compositions in which the ratio between sialic acid containing oligosaccharides and phosphorus, and preferably the ratio between sialyllactose and phosphorus, is relatively high. In addition, the invention relates to the obtainable and obtained compositions and the use of those compositions, in especially infant foods.
Sialyllactose is a milk oligosaccharide which occurs in milk of mammals. It is known that the concentration of this milk oligosaccharide in, for instance, cow's milk is very low; in human milk the sialyllactose occurs in concentrations which in an absolute sense are considerably higher.
Sialyllactose has a number of important functionalities. Thus, it is known that it possesses a prebiotic activity, and bifidogenic effects have been described. In addition, the sialyllactose is an anti-adhesive, allowing it to play an important role in inhibiting or preventing intestinal infections by binding pathogenic bacteria and/or viruses and/or toxins being released, so that colonization on the one hand and the effect of the toxins on the other hand can be prevented. Also, it has been described that sialyllactose can promote brain growth in infants.
Such positive effects are also attributed to other sialic acid containing oligosaccharides.
Since, as a rule, the aim is to make infant and baby foods resemble human milk as much as possible, there is a need (especially with infant milk formulators) to enrich such foods with sialic acid containing oligosaccharides and preferably with sialyllactose. This is also one of the objects of the present invention.
To obtain sialic acid containing oligosaccharides in general and sialyllactose in particular, different processes have already been proposed in the prior art. A large number of those processes are aimed at the enzymatic preparation of sialyl group containing compounds such as sialyllactose, and especially using enzymes obtained from genetically modified organisms, whereby, with for instance a sialidase, sialyl transferase and/or neuraminidase, the desired sialyl group containing oligosaccharides can be formed and which can then be isolated through downstream processing.
In addition, a number of processes are known whereby sialic acid and sialyllactose are isolated from milk through combinations of chromatographic techniques and/or membrane filtration techniques.
An important disadvantage of the existing methods of recovering sialyloligosaccharides and sialic acid is that they yield the desired compounds in relatively low contents, or concentrations, while moreover in these methods a high phosphorus content is present in the sialyloligosaccharide composition. Publications are known in which it is described that foods that are phosphorylated to a high degree are digested poorly, at least less well, may adversely affect the calcium and iron absorption into the system, and generally complex with bivalent cations that are necessary for usual physiological processes in the gastrointestinal tract, while moreover the action of certain enzymes in the gastrointestinal tract may be inhibited. Especially with infant and baby food, this can lead to problems.
In an infant milk formula, the amount of phosphorus is determined especially by the protein additions. Extra phosphorus additions through other ingredients are not recommended and even advised against. Also for this reason, the provision is contemplated of sialic acid containing oligosaccharide compositions and in particular sialyllactose containing compositions in which the ratio between sialyl groups and phosphorus is as high as possible.
An object of the process according to the present invention is to provide a process in which separation techniques are used that lead to a product with relatively few phosphorus compounds in relation to the sialic acid containing oligosaccharide. This makes the product highly suitable to be used in infant foods.
Such a process is also described in U.S. Pat. No. 6,623,954. In this publication a process is described in which a dairy stream containing lactose and sialyloligosaccharides is contacted with phytase and a β-galactosidase, after which the dairy stream is subjected to nanofiltration and/or ultrafiltration, whereby the desired sialyllactose is retained in the peptide fraction; after a denaturation step and a subsequent microfiltration, a sialyllactose fraction is obtained. Optionally, before the step in which the dairy stream is contacted with the enzymes, a step may be conducted in which the positively charged materials are removed, for instance with a cation exchanger or with reverse osmosis.
An important disadvantage of this process is that as a result of the membranes used an important part of the sialyloligosaccharides including sialyllactose cannot be recovered and that it may take a long time before a product is obtained that satisfies a low ratio of phosphorus to sialyloligosaccharides.
The present invention is carried out without it being necessary to use enzymes to obtain a low phosphorus content. Further, the invention makes it possible to recover sialyloligosaccharides with a high degree of efficiency (high yield) and selectivity (high purity).